More than fifteen million interior door/jamb systems are installed in the United States alone each year. The great majority of these doors are supplied to the job site as a "pre-hung door". A typical pre-hung door takes approximately 60-90 minutes for a skilled carpenter to install and involves plumbing, shimming and attaching the door jamb to wall studs, attaching trim around the door opening to hide the gap between the jamb and the wall, and finally hinging the door to the jamb and installing the hardware.
The most difficult and time consuming part of installing a pre-hung door is squaring and plumbing the door jamb so that the door can freely swing between opened and closed positions and remain at rest in any single open position. Others have attempted to provide mechanisms for simplifying the jamb squaring and plumbing process.
For example, Putnam Products of Old Saybrook, Connecticut offers a tool called a "JAMJIG.TM." which supposedly holds the jamb in square alignment so that the installer's hands are free to shim and fasten the jamb to the rough opening. Although tools such as the JAMJI.TM. may be helpful in partially simplifying the plumbing process, it is an object of the present invention to completely eliminate the need for on-site squaring, plumbing and shimming of the jamb. It is also desirable to decrease or eliminate the need for special tools or special carpentry expertise on the job site.
Others have attempted to simplify door installation by proposing partially preassembled door/jamb systems. For example, each of U.S. Pat. Nos. 3,707,057, to Frydenberg, 3,599,373 to Coykendall, 2,711,564 to Jackson and 2,736,930 to Longley, disclose a partially preassembled split jamb system with an adjustment mechanism for varying the collective width of the jamb to accommodate differences in wall thickness. However, the door frame assemblies disclosed in these patents require plumbing of the jamb legs and attachment of the header portion of the jamb during installation. None of the door/jamb systems disclosed in these patents are completely prefabricated. They all require some assembly of jamb and trim components during installation at the job site and they all require elaborate mechanisms for attaching the jamb to the wall.
Another problem with the prior devices is that the split/jamb components are attached to each other during installation. Therefore, after the unit is installed the jamb and trim assembly is not capable of accommodating shrinkage or expansion of the wall width.
It has apparently been a fundamental concept in prior door/jamb systems to rely on the wall structure to support and maintain the square orientation of the door jamb throughout the life of a swinging door. As noted by Jackson in U.S. Pat. No. 2,711,564, column 1, lines 43-48:
The support of jamb members by trim strips has not been found satisfactory in that the jamb may shift from the weight of the suspended door or from warping of the jamb or trim strips, or from other causes. It is important, therefore, rigidly to affix the jamb of the door casing to the rough buck of the rough door opening.
Thus, prior innovators have focused their efforts on improving the methods of attaching a jamb to a wall. However, regardless of the attachment mechanism, in order to make the connection, it is inherently necessary to vertically align the jamb, i.e., shim and plumb the legs of the jamb, relative to the wall and floor. Nobody has previously been able to produce a truly prefabricated unit which is pre-squared and pre-plumbed independently from the wall and is capable of standing independently and supporting a swinging door without attachment to the wall.
Another problem with door jambs being attached to the wall, is that the shape, angles or dimensions of the wall opening are different from site to site, and may change after the door is installed, for example, due to wood shrinkage, settling, earthquakes or other natural causes. If the door jamb and trim are anchored to the wall, then changes in the wall opening dimensions will cause corresponding shifts in the door frame resulting in door functioning problems as well as flaws in the appearance of the door, jamb and trim.
Regardless of what the reasons are, previously disclosed "prefabricated" door/jamb units are not currently being used with any significant frequency, if at all in the construction industry.